A typical transmission link of an optical fiber communication system contains various optical amplifiers, in particular various erbium doped fiber amplifiers (EDFAs), which may play different roles depending on their location in the link. Accordingly, the link may have multiple types of EDFAs with differing configurations and/or specifications. A typical EDFA specification includes target values or ranges for such parameters as ADFA gain (G), noise figure (NF), gain tilt and gain ripple. All these parameters may have different target ranges depending on a position of the EDFA in the link, thereby necessitating differing amplifier designs.
By way of example, FIG. 1 schematically illustrates a portion of a conventional transmission link of a long-haul optical network, in which an EDFA 9-1 is a booster amplifier that boosts the power of an optical signal from an optical signal transmitter 6 to a target optical power for launching into a transmission span 13-1. Typically, EDFA 9-1 is a single stage amplifier that has only two optical connections, i.e. input and output. EDFA 9-2 compensates the loss in the optical signal from the transmission span; and also compensates dispersion, caused by the transmission fiber, by passing the signal through a dispersion compensation unit (DCU) 8-1. EDFA 9-2 is typically referred to as a dual stage amplifier or an amplifier with a mid-stage access. The DCU 8-1 is considered to be connected between gain stages or “at midstage”. The EDFA 9-2 has at least four optical connections: amplifier input, amplifier output, midstage input and midstage output. EDFA 9-3 is similar to EDFA 9-2 as it compensates for an optical loss and chromatic dispersion in transmission fiber, and is also a two stage amplifier with midstage access. DCU 8-2 may have different characteristics, or even may be based on a different technology. Furthermore, loss-span compensation, gain flatness and noise figure requirements to the EDFA 9-3 can be significantly different from requirements to EDFA 9-2, as well. EDFA 9-4 and EDFA 9-5 are single stage booster amplifiers located after Reconfigurable Optical Add/Drop Module (ROADM) 7, and while they are similar to EDFA 9-1 in function and the number of amplification stages, they are located in different part of the transmission link, and as a result gain, noise and flatness requirements to EDFA 9-4 and 9-5 can be quite different from that of EDFA 9-1.
Thus, different optical amplifiers may have different specifications and require different number of stages and connections. While some of the amplifier parameters can be adjusted in conventional amplifiers, such as amplifier gain, such adjustment is limited in range due to its influence on other parameters, for example reducing the gain of an EDFA typically decreases the noise figure, while increasing the gain beyond a design-dependent optimal value may adversely affect the gain flatness or tilt.
A conventional way of dealing with this problem is to fabricate different amplifiers in a variety of configurations designed to different specifications. To illustrate this, FIG. 2 shows different exemplary NF-Gain characteristics of the four EDFAs in FIG. 1. As an example, EDFA 9-1 could be of configuration 1, EDFA 9-2 could be of configuration 3, EDFA 9-3 could be of configuration 4, and EDFAs 9-4 and 9-5 could be of configuration 2. In other situations and for other portions of the network, the number of EDFA configurations as well as their characteristics can be again different.
A drawback of such approach is that it requires having stacks of different amplifiers while building and servicing optical communication links, which greatly increases the costs of building and maintaining the network.
The problem is further exacerbated by a high cost and complexity of conventional EDFAs, which includes a multitude of optical and electronic components of different types. A typical prior-art EDFA includes one or more coils of erbium-doped fiber (EDF) as the gain medium, semiconductor lasers to pump the EDF, and discrete fiber-coupled components such as optical taps and WDM couplers, optical isolators, gain flattening filters (GFFs), and variable optical attenuators (VOAs), to properly couple and guide signal light and pump light. Fiber-coupled photodiodes (PDs) are used to measure input and output optical power levels. Fiber splicing is used to optically couple the components together. As a result, a typical prior-art EDFA has numerous fiber splices, splice protectors, discrete components, and optical fiber loops. The multitude of components and fiber loops make the conventional EDFAs complex and costly. Using prior-art technologies and approaches, reducing amplifier costs requires sacrificing EDFA performance characteristics such as the spectral gain tilt, flatness of the gain spectrum, and the noise figure of the EDFA, which is undesirable from the standpoint of maintaining a high level of technical performance.
US patent application 2009/0201576, which is referred to hereinbelow as the '576 application, has common inventors with the present application, is assigned to the assignee of the present application, and is incorporated herein by reference, discloses an EDFA build with the use of planar lightwave circuit (PLC) technology, wherein most of the EDFA components such as optical taps, pump splitters, optical isolators, monitoring PDs are build in or mounted on a PLC chip, and the EDF and pump diode lasers are coupled to the PLC by a fiber array, eliminating most of the fiber pigtails used in conventional EDFAs. This PLC-centered approach enables to reduce the size, cost and fabrication complexity of conventional EDFAs without sacrificing their performance.
Furthermore, the PLC-centered approach greatly reduces the cost of adding new components, enabling also use novel components not conventionally used in EDFAs, such as tunable spectral tilters and tunable optical power splitters, as described in the '576 application. The use of such variable splitter as a variable pump splitter in a PLC-centered EDFA of the '576 application enables an independent control of optical pumping of two EDFA stages using a single pump laser diode, reducing the amplifier cost or improving its characteristics as compared to conventional EDFAs.
An object of the present invention is to further improve upon the prior art optical amplifiers by providing an optical amplifier that is reconfigurable for use in a wide range of network requirements and locations, and is relatively simple to assemble.